jtag_vpi: added an option to stop simulation on exit

jtag_vpi: added an option to stop simulation on exit

Command CMD_STOP_SIMU had been defined in jtag_vpi for a long time
(since the beginning?) but has not been utilized until now.
Its purpose is to signal to the jtag_vpi server (i.e. the RTL
simulation software) that the simulation shall be stopped.

This commit adds a TCL configuration command that selects whether
CMD_STOP_SIMU will be sent to the jtag_vpi server when OpenOCD is
about to exit. This functionality is off by default to maintain
identical behavior as in previous OpenOCD versions, unless the user
enables it explicitly.

Change-Id: If3894af6efa61038ccf6c9191f664e2128f2ef11
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/5407
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>